Atomic layer deposition (ALD) of polyimide (PI) is explored to tune the separation properties of microporous polyethersulfone (PES) membranes and also to improve their mechanic and thermal stability. Conformal and uniform thin layers of PI are deposited along the pore wall throughout the entire PES membrane instead of forming a top layer merely on the membrane surface. With increasing ALD cycles, the pore size of the PES membrane is progressively reduced, leading to increased retention. The permeation is correspondingly decreased but its drop is less pronounced than the increase of retention. For example, the retention to 23-nm silica nanospheres is significantly increased from nearly zero to 60% after 3000 ALD cycles, whereas the water flux is moderately decreased by 54%. Moreover, ALD of PI evidently enhances the mechanical strength and thermal resistance of the PES membrane as PI tightly wraps the skeleton of the membrane. (c) 2014 American Institute of Chemical Engineers AIChE J, 60: 3614-3622, 2014